Master 62 metadata link support

Here some proposal for the Metadatalink support. I've just started on github, my apologies about the diff, for some unknown reason, sometimes the diff is readable (see 2d commit), and sometimes the diff considers the entire file. If you have an idea on how to solve that, it is welcome! I've tried to split in a few commits to avoid such issue, but as you can see it didn't work for all commits. 

I give here a summary:

Metadatalink encoding
- new package it.geosolutions.geoserver.rest.encoder.metadatalink that handles a new class GSMetadataLinkInfoEncoder, and a enum ResourceMetadataLinkInfo
- GSResourceEncoder: new methods to encode metadatalinks

Metadatalink decoding
- RESTResource: new methods to decode metadatalinks

Tests for encoder/decoder

Kind regards,
Emmanuel